On January 21st, 2011, Thomas Geernaert successfully defended his PhD thesis and was awarded the degree of Doctor in Engineering. His PhD research focused on "Microstructured fiber Bragg grating sensors: from fiber design to sensor implementation" (promotors: prof. dr. ir. Francis Berghmans and prof. dr. ir. Hugo Thienpont).
The goal of this work was to design, simulate, fabricate, characterize and implement a temperature-insensitive optical fiber sensor for hydrostatic pressure and transverse strain.
The use of sensors in robotics, material manufacturing, civil construction and healthcare becomes more and more important for collecting data on the influence of physical quantities. These quantities typically include strain, temperature, pressure or force. In many cases conventional electromechanical sensors, such as electrical strain gauges, are perfectly qualified for these tasks. Whereas the simplicity of these conventional sensors is definitely an advantage, their application range is still limited due to important drawbacks such as their disturbing cross-sensitivity to physical quantities that are not of interest for the measurement, the difficulty to multiplex these sensors and their size that does not allow integrating the sensors in a non-intrusive manner inside different materials. When traditional sensors fail optical fiber sensors can provide a solution.
Optical fiber sensors are small and lightweight micro-optic sensors that exploit the characteristics of an optical signal propagating inside an optical fiber. Their implementation and commercial success has considerably increased in the last 25 years. In spite of their very long list of advantages, they may still require complex temperature cross-sensitivity correction systems that increase the overall system cost and that impede their easy integration inside materials. In this thesis we overcome this hurdle by developing a temperature-insensitive pressure sensor that combines a new generation of optical fibers – the so-called microstructured optical fibers – with fiber Bragg grating technology. By carefully designing the topology of the array of microscopic air holes that run along the entire length of these fibres, we tailor the sensing properties of a Bragg grating, fabricated inside this optical waveguide, with an unprecedented freedom and precision. We succeed in developing a new optical fiber sensor that combines temperature-insensitivity with an order of magnitude increase in pressure and transverse load –sensitivity.
Owing to this development, our sensors are ready to be applied for pressure monitoring in the field of oil and gas exploration and for structural health monitoring inside composite materials in the field of aeronautics.
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